Prevention of a rotor overspeed condition in wind turbines is an important consideration in designing a commercial wind turbine for reliable operation. Under normal operating conditions, the rate of energy capture by the aerodynamic rotor assembly of a wind turbine is balanced by the rate of conversion to electrical energy in the generator, resulting in controlled shaft speed. However, a number of conditions may occur which render this control inadequate, if a fault occurs in the generator or associated power conversion apparatus, the electromechanical load on the rotor assembly may be suddenly lost, resulting in rapid acceleration of the rotating apparatus to unsafe speeds. Alternatively, very high winds may occur, resulting in input power in excess of the power rating of the generator.
Turbines are typically designed to limit aerodynamic input power and rotor speed in high winds. Large megawatt-scale turbines typically accomplish this by pitching the blades. Home-scale turbines often limit power in high winds by arranging for the entire rotor assembly to rotate passively away from the incoming flow—termed furling.
An alternative means of limiting power and preventing overspeed is to modulate the torque of the generator to induce aerodynamic stall in high winds. This method is used on small commercial turbines, but renders the turbine vulnerable to failures in the generator, which may result in an uncontrolled overspeed condition and likely structural failure. On larger turbines, active, releasable mechanical brakes are employed to back up the primary control method, but conventional mechanical brake assemblies may be cost prohibitive on smaller turbines.